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Acta Physica Sinica

OBSERVATION OF INTENSITY SQUEEZING AND LARGE PHASE NOISE REDUCTION IN SELF-LOCKED SEMICONDUCTOR LASERS

Tiancai Zhang, 李天宇 Li Tianyu, D. van Effenterre, Changde Xie, Kunchi Peng, (1)Ecole Polytechique,Palaiseau,91128,France; (2)山西大学光电研究所，太原 030006

Published Jan 1, 1998• 3 citations
Publisher

Abstract

Both intensity squeezing and dramatic phase noise reduction have been obtained with a confocal cavity used to feedback the beam from the quantum well diode laser.We have measured an intensity squeezing of 19.3%(37.8% inferred at the laser output) around 35MHz and achieved phase noise reduction of 40—25dB from 9kHz to 100MHz.Quasi-minimum uncertainty state have been demonstrated.

DOI
10.7498/aps.47.1498

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Both intensity squeezing and dramatic phase noise reduction have been obtained with a confocal cavity used to feedback the beam from the quantum well diode laser.We have measured an intensity squeezing of 19.3%(37.8% inferred at the laser output) around 35MHz and achieved phase noise reduction of 40—25dB from 9kHz to 100MHz.Quasi-minimum uncertainty state have been demonstrated.
We have investigated the intensity and phase noise of single-mode laser diodes, either free-running or using different types of line narrowing techniques at room temperature, namely feedback from an external grating and injection locking. We have measured an intensity squeezing of 1.2 dB in the first case, and 1.4 dB in the second case (respectively, 1.6 dB and 2.3 dB inferred at the laser output). We have observed that the intensity noise of a free-running 'single-mode' laser diode actually results from a cancellation effect between large anticorrelated fluctuations of the main mode and of weak longitudinal side modes. It is also shown that free-running diodes exhibit very large excess phase noise. Typically more than 80 dB above shot noise at 10 MHz, which can be significantly reduced by the above-mentioned techniques.
The quantum correlation between junction voltage fluctuations1/frequency fluctuations2 and the intensity fluctuations of a semiconductor laser may be used to reduce the intensity noise. Here we address squeezed-state generation discussing both fundamental and practical limitations on noise reduction in conventional semiconductor lasers and microcavity lasers. In conventional semiconductor lasers, starting with intensity noise at the standard quantum limit, ideally a 3-dB amplitude squeezing is possible. This fundamental limit is set by spontaneous emission noise. Another fundamental limit is that noise reduction beyond the level set by optical losses and pump noise is not possible. It is also shown that the number-phase uncertainty in some conditions can reach the ultimate limit set by Heisenberg’s uncertainty principle. These conditions are clarified and a comparison made between junction voltage and frequency noise feedforward. Feedforward schemes realizing intensity noise reduction are discussed.
The quantum noise properties of injection-locked semiconductor laser oscillators are investigated. Included in the analysis are pump-noise suppression and squeezing of the injection signal. It is shown that the main effect of injection locking is the reduction of the phase noise of the outgoing laser light field. The influence that squeezing of the injection signal state has on the output state is shown to be strongly dependent on the value of the linewidth enhancement factor. In general, little or no noise reduction stands to be gained by squeezing the injection signal. It is also shown that under idealized conditions, the spectral uncertainty product of the outgoing field will approach, but never go below, the minimum uncertainty value given by the Heisenberg uncertainty relation.
This thesis describes the phase and amplitude noise properties of semiconductor lasers subjected to weak, dispersive optical feedback. In the first half, experiments demonstrating reductions in the laser frequency noise power spectrum and spectral linewidth by several orders of magnitude are presented. Weak optical feedback is applied to the laser by an external cavity containing an atomic vapor. The presence of the vapor adds to the dispersion of the cavity and simultaneously locks the laser to a fixed frequency reference. The role of 1/f frequency noise in limiting the effectiveness of this linewidth reduction technique is investigated and 1/f noise is found to be the dominant contribution to the linewidth under strong optical feedback conditions. An electronic feedback scheme utilizing FM sideband locking is then implemented alongside the optical feedback, and an additional reduction in the low frequency 1/f frequency noise power spectrum by over two orders of magnitude is obtained. With both systems operating simultaneously, the spectral linewidth is narrowed from its free-running value of about 20 MHz to 1.4 kHz. Excellent absolute frequency stability is also achieved. In the second half, the effects of optical feedback on the quantum mechanical amplitude noise properties of the laser are examined. A fully quantum mechanical theory of amplitude and phase noise for a semiconductor laser with weak optical feedback is developed, and the nature and limits of the noise reduction using this technique are established. Particular attention is given to the feedback-induced enhancement of the amplitude squeezing which can be obtained in a pump-suppressed semiconductor laser: an improvement in the squeezing by 3 dB is predicted under moderate pumping. Somewhat larger noise reductions are expected when the laser is operating closer to threshold. Measurements performed on a laser biased close to threshold are then described and a reduction in the low frequency amplitude noise power spectrum by 7 dB is obtained. An experimental investigation of the effects of optical feedback on the amplitude squeezing in a semiconductor laser is then discussed. The low frequency squeezing in a room temperature device is increased from 3% below the standard quantum limit (SQL) under free-running conditions to 19% below the SQL with optimal feedback. The experimental results are found to agree poorly with the single-mode model and a multi-mode model including the effects of asymmetrical cross-mode non-linear gain is developed to explain the discrepancy. Finally, further experimental investigation into the generation of amplitude squeezed light is presented using a commercial laser with no intentional external modifications. Squeezing as large as 29% below the SQL is measured using a balanced homodyne detector with the laser operating near room temperature, corresponding to 41% below the SQL at the output facet of the laser.
Diode lasers are widely used in atomic physics given their narrow linewidth and wavelength tunability. Nevertheless, although they present low noise for their intensity, excessive noise in the phase limits their application in quantum optics. Looking for reduction of this phase noise, we built and characterized a ring laser, using a semiconductor-tapered amplifier as the gain medium. We were able to reduce the phase noise of a diode laser to a factor of 10 above shot-noise level, bringing it closer to a useful coherent state for applications in quantum information.
Number states of the electromagnetic field are ideal for optical communication and for a variety of high precision measurements. It has been shown theoretically that the light from a semiconductor laser can opproach a number state1. In previous experiments2,3 the observed amplitude noise reduction, below the standard quantum limit, was at most 1.5 db. In this experiment we have been able to reduce the effects of optical feedback, increase the collection efficiency and consequently further elucidate the basic device physics necessary for improvement in the noise reduction. We have observed that the measured squeezing is not limited by the laser overall quantum efficiency but only by the optical output coupling efficiency. An explanation is advanced based on the premise that current division does not introduce Poissonian noise but thermal noise.